A unique condition for early diversification of small shelly fossils in the lowermost Cambrian in Chengjiang,South China: Enrichment of phosphorus in restricted embayments |
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| Institution: | 1. Department of Earth Science and Astronomy, University of Tokyo, Tokyo 153-8902, Japan;2. Early Life Institute, Northwest University, Xi''an 710069, China;1. Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan;2. Mineral Resource Research Group, Institute for Geo-Resources and Environment Geological Survey of Japan, Japan;3. Department of Geology and Mineralogy, Division of Earth and Planetary Sciences, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan;4. Department of Geology and Key Laboratory for Continental Dynamics, Northwest University, Xi''an 710069, China;5. Department of Earth Science and Astronomy, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan;1. Early Life Institute and State Key Laboratory of Continental Dynamics, Northwest University, Xi''an 710069, PR China;2. Department of Earth Science and Astronomy, University of Tokyo, Tokyo 153-8902, Japan;3. Earth-Life Science Institute, Tokyo Institute of Technology, Tokyo 152-1551, Japan;1. State Key Laboratory of Continental Dynamics, Department of Geology, Northwest University, 229 Taibai Road, Xi''an 710069, PR China;2. Department of Geological Sciences, University of Missouri, 101 Geology Building, Columbia 65211, MO, USA;3. Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo, Japan;4. Tohoku University Museum, Tohoku University, 6-3 Aoba, Aramaki, Aoba-ku, Sendai, Japan;5. Department of Earth Science and Astronomy, Graduate School of Arts and Sciences, The University of Tokyo, Tokyo 153-8902, Japan |
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| Abstract: | In the earliest Cambrian the major diversification of small shelly fossils (SSFs) was the first episode of the so-called Cambrian Explosion. In order to clarify the background environmental conditions of this event, we examined the lowermost Cambrian strata with bedded phosphorites in the Chengjiang area, South China. The lowermost Cambrian (the Zhongyicun Mb of the Zhujiaqing Fm) in eastern Yunnan is composed of bedded phosphorites and phosphatic limestones with diverse SSFs. We investigated 3 sections within the Chengjiang area at Hongjiachong, Maotianshan, and Xiaolantian. Detailed lithostratigraphic analysis of outcrops and drill cores at Hongjiachong indicates that the Zhongyicun Mb consists of 5 distinct units, A to E in ascending order. The presence of 15 genera of SSFs in 20 horizons shows that the Zhongyicun Mb yields two distinct SSF assemblages of Fortunian age (earliest Cambrian; 541–529 Ma); i.e. the first assemblage with simple-shape SSFs (Anabarites, Protohertzina) from basal Unit A, and the second assemblage with various molluscan shells (Paracarinachites, Ocruranus–Eohalobia) from Units C–E. As well as abundant phosphate grains, all SSFs occur as clastic grains, suggesting that phosphorite was primarily formed in extreme shallow-water settings, as were the small shelly animals. We established that the first occurrence of the second SSF assemblage is at least 5 m lower (ca. 1–2 myr earlier in age) than previously reported in Chengjiang, and we speculate that the major diversification in SSF assemblage likely occurred during the Fortunian, at least before ca. 534 Ma. Judging from the rift-related tectonic setting and relevant paleogeography of western South China, we further speculate that the Zhongyicun Mb was primarily deposited in restricted embayments in the Kangdian rift basin, and that the rapid SSF diversification in the Fortunian occurred in a unique setting in highly phosphorus-rich seawater. |
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